Inhibitory effect of sodium nitroprusside on the relaxing action of isoproterenol in isolated rat urinary bladder

Enhanced alveolar fluid clearance following 72 h of continuous isoproterenol infusion in rats

AIM

We wished to determine the effect of continuous β-receptor stimulation on alveolar fluid clearance and to elucidate the mechanisms behind this effect.

METHODS

Alveolar fluid clearance was measured in anaesthetized rats pretreated for 72 h with the β-agonist isoproterenol (200 μg kg(-1) h(-1) sc) or vehicle. Alveolar fluid clearance in artificially ventilated rats was determined over 1 h by infusion of isotonic Ringer solution containing (125) I-albumin into the lungs. Additionally, alveolar fluid clearance was determined when amiloride or l-cis-diltiazem was added to the solution to block ENaC and cyclic nucleotide-gated (CNG) channels respectively.

RESULTS

Isoproterenol treatment induced a 42% increase in alveolar fluid clearance (18.9 ± 1.4%) vs. vehicle (13.3 ± 3.3%). Addition of amiloride resulted in a net decrease of 8% in both groups, while l-cis-diltiazem caused a net decrease of 12% in isoproterenol-treated animals, but only 5% in vehicle-treated animals. Western blotting showed that isoproterenol treatment increased the abundance of the α-ENaC and β-ENaC subunits (223 ± 51% and 274 ± 55% of vehicle, respectively) but we saw no changes in protein level of the γ-EnaC subunit.

CONCLUSION

Continuous β-adrenoceptor stimulation with isoproterenol enhances alveolar fluid clearance through alternative pathways involving l-cis-diltiazem-sensitive channels.

Cyclic nucleotide phosphodiesterases in rabbit detrusor smooth muscle

OBJECTIVES

To identify the phosphodiesterase (PDE) isoenzymes in the rabbit detrusor and to evaluate their roles in regulating detrusor muscular tone. Cyclic nucleotides are important secondary messengers involved in modulating the contractility of various smooth muscles. Cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are synthesized by their respective cyclases and degraded by various PDEs.

METHODS

PDE isoenzymes from male and female rabbit detrusor were isolated by the Mono-Q anion exchange column and identified with various inhibitors. Detrusor strips from both sexes were precontracted with carbachol and relaxed with PDE inhibitors and adenylate and guanylyl cyclase activators in a tissue bath. Cyclic nucleotide concentrations in strips from male rabbits were determined after the compound treatment.

RESULTS

Similar results were obtained from both sexes in the experiments in which both sexes were used. The activities of PDE1, 2, 3, 4, and 5 were identified. Forskolin induced a dramatic rise in the cAMP levels and was the most effective relaxant. Papaverine generated moderate increases in the cAMP and cGMP levels and induced very good relaxation. Vinpocetine produced no detectable changes in the cyclic nucleotide levels but elicited good relaxation. Sildenafil caused an increase in the cGMP levels and had a similar relaxation effect as vinpocetine. Sodium nitroprusside induced some increase in cGMP and had no relaxation effect. Rolipram raised the cAMP levels significantly, yet had a moderate effect on relaxation.

CONCLUSIONS

Our results demonstrated the presence of PDE1 through 5 in rabbit detrusor muscle and supported their involvement in regulating detrusor muscle tone. The relaxation of rabbit detrusor was mainly mediated by the cAMP pathway.

Contribution of amiloride-insensitive pathways to alveolar fluid clearance in adult rats

The contributions of amiloride-sensitive and -insensitive fractions of alveolar fluid clearance in adult ventilated rats were studied under control conditions and after beta-adrenergic stimulation. Rats were instilled with a 5% albumin solution containing terbutaline (10(-4) M) or dibutyryl-cGMP (DBcGMP; 10(-4) M) with or without the cyclic nucleotide-gated cation channel inhibitor l-cis-diltiazem (10(-3) M) and/or amiloride (10(-3) M). Alveolar fluid clearance over 1 h was 18 +/- 2% in controls. In controls, amiloride inhibited 46 +/- 15% of alveolar fluid clearance, whereas l-cis-diltiazem had no inhibitory effect. Terbutaline and DBcGMP stimulated alveolar fluid clearance by 85 +/- 3 and 36 +/- 5%, respectively. Amiloride and l-cis-diltiazem inhibited nearly equal fractions of terbutaline-stimulated alveolar fluid clearance when given alone. Amiloride and l-cis-diltiazem given together inhibited a significantly larger fraction of alveolar fluid clearance in terbutaline-stimulated rats and in DBcGMP-stimulated rats. Based on these data, terbutaline stimulation recruited both amiloride-sensitive and l-cis-diltiazem-sensitive pathways. In contrast, DBcGMP mainly recruited l-cis-diltiazem-sensitive pathways. Therefore, the amiloride-insensitive fraction of Na+-driven alveolar fluid clearance may be partly mediated through cyclic nucleotide-gated cation channels and activated by an increase in intracellular cGMP.

Potentiation of the relaxing action of isoproterenol by forskolin in rabbit aortic rings: the involvement of beta 2-adrenoceptors

1. Forskolin, an activator of adenylate cyclase, potentiated the relaxing response to isoproterenol in rabbit aortic rings precontracted by phenylephrine (PE). 2. The potentiating effect of forskolin was inhibited by propranolol, a beta-adrenoceptor inhibitor, but not by methylene blue, a guanylate cyclase inhibitor. 3. The relaxing response to terbutaline, a beta 2-adrenoceptor agonist, but not lower concentrations of dobutamine, a beta 1-adrenoceptor agonist, also was potentiated by forskolin. Forskolin, however, potentiated the relaxing response to high concentrations of dobutamine, which activates both beta 1- and beta 2-adrenoceptors. 4. Yohimbine, an alpha 2-adrenoceptor inhibitor, glyburide, an ATP-sensitive K+ channel inhibitor, iberiotoxin, a Ca(2+)-activated K+ channel inhibitor, or endothelium-removal failed to affect the potentiating effect of forskolin. 5. Dibutyryl cyclic AMP (cAmp) also potentiated the relaxing response to terbutaline. 6. These results suggest that in rabbit aortic rings forskolin causes the apparent potentiation of isoproterenol-induced relaxation by mainly affecting the relaxing response due to the activation of beta 2-adrenoceptors by the forskolin-induced increase in the level of cAMP.